Various types of impact sensors and impact sensing systems are used in automotive passive passenger restraint mechanisms such as airbags or automatic seat belt tensioners. These impact sensing systems are designed to sense high vehicle impact conditions and determine if and when activation of a passenger restraint is required for a given impact condition. Discrimination between activation worthy and non-activation worthy events ensures that the passenger restraint is activated only when necessary.
Most impact sensing systems can be placed in one of two categories. One type of system uses a plurality of mechanical threshold switches in the front region of the vehicle. These switches close if the vehicle experiences a frontal impact which is severe enough to close the switches. Mechanical sensor-based systems of this type often rely on sensor redundancy to reduce any negative effects of sensor malfunction which may occur. This requires a large number of switches which must be individually tested and strategically placed in the vehicle to ensure proper operation of the entire impact sensing system. These sensing systems can only be activated by mechanical accelerations having the magnitude and duration under which they were designed to operate in a crash. Thus, there is no practical way to test switch response after installing the switches in the car, since the switches can only be activated by large mechanical forces.
Single-point impact sensing systems rely on an accelerometer located in the passenger compartment of the vehicle to constantly monitor the acceleration and sense any sudden deceleration of the vehicle. The output of the accelerometer is continuously analyzed and discriminated to determine if and when any unusual decelerations occur, indicative of vehicle impact. The passenger restraint is activated if the output of the accelerometer senses a severe deceleration which is indicative of an activation worthy event.
Since single-point impact sensing systems do not require the large number of components that mechanical impact sensing systems have, diagnostic circuitry for testing a single-point sensing system can be simplified. System testing on the vehicle after installation involves checking the operation of the accelerometer and the discrimination circuitry rather than a large number of mechanical switches.
One type of impact sensor testing system is described in U.S. Pat. No. 4,359,715 to Langer et al. A diagnostic module is electrically connected to several different electrical locations in the impact sensor to monitor the voltage levels and detect the occurrence of faults at those locations. Upon detection of a fault, a warning lamp displays a signal which is unique depending on the fault type or location which occurred in the impact sensor. The diagnostic module checks for open circuits, short circuits and faulty components within the impact sensor. This type of diagnostic system however, only checks for electrical continuity within the sensing system and proper operation of individual sensor components. No provisions are made in this device to check for gradual changes such as drift or loss of calibration in the sensor.
Another circuit for testing the operation of an impact sensor for use with airbags is described in U.S. Pat. No. 4,243,971 to Suchowerskyj et al. A testing program is initiated when electrical power is applied to the passenger restraint system. The program checks the operation of the discrimination circuits by generating a simulated trigger signal at the discriminator input. Proper operation is confirmed if the output power transistor in the output stage of the impact sensor becomes conductive. The output stage is also tested for electrical continuity to ensure that the airbag will be deployed during a high impact condition. Changes in the calibration of the sensor would not be detected in this arrangement. Additionally, the duration of the trigger signal is considered to be critical to proper testing, requiring careful timing considerations which can be adversely affected by regular operating conditions. This criticality in timing is not desirable for a diagnostic circuit since changes in temperature or operating environment can change the response of the circuit.
U.S. Pat. No. 5,060,504 to White et al. describes a method for self-calibrating an accelerometer. The output of an electrical control circuit for the accelerometer is periodically calibrated by electrostatically displacing a sensing mass in the accelerometer relative to a frame. The displacement caused by the electrostatic force is analogous to a displacement caused by a known acceleration. The resultant change in output in the control circuit is used as a reference value for calibrating subsequent output changes caused by acceleration. The electrostatic displacement of the sensing mass in the accelerometer is used to correct calibration changes within the accelerometer, but there is no circuitry which can be adapted to test the functioning of the accelerometer in conjunction with associated discrimination circuitry to correctly sense an activation worthy event.